1. Field of the Invention
The present invention relates to an electric discharge machining method of machining or cutting an insulating material to a predetermined shape using an electroconductive layer formed thereon to promote electric discharge between the insulating material and a work electrode.
2. Description of the Prior Art
Since insulating materials such as ceramics show electrically insulating property, electric discharge necessary for machining does not occur. In order to electric discharge machine such an insulating material, it is necessary to form an electroconductive layer on the surface of the insulating material to be machined.
For instance, Japanese Patent Application Laid-Open 63-15010 discloses the method wherein electric discharge is generated between electrodes so as to deposit carbon particles or the like from a machining liquid. The deposited carbon is adhered onto the surface of an insulating material, or the insulating material is impregnated with the deposited carbon. Hereby, an electroconductive layer is formed on the surface of the insulating material.
Such an electroconductive layer useful for electric discharge machining may be formed by coating the insulating material with an electroconductive film, as disclosed in Japanese Patent Application Laid-Open 4-41120
According to the electric discharge machining method disclosed in Japanese Patent Application Laid-Open 63-150109, an insulating material 1 is coated with an electroconductive layer 2 such as Cu or Fe, as shown in FIG. 1(A). The electroconductive layer 2 may be fomed by flame spraying or vapor deposition to 0.1-0.5 mm in thickness. The insulating material is then mounted on the table of an electric discharge machine, and secured onto the table with proper clamping means.
The insulating material is dipped in a machining liquid such as oil, and a potential for electric discharge machining is charged between the electroconductive layer 2 and a machining electrode 3 from a power source to start electric discharge machining, as shown in FIG. 1(B). At first, the electroconductive layer 2 is machined by the electric discharge, as shown in FIG. 1(C).
When the electroconductive layer 2 is removed at a part facing to the machining electrode 3, a fresh electroconductive layer 4 is formed on the exposed surface of the insulating material 1. The electroconductive layer 4 is caused by the deposition of machined chips formed by electric discharge machining and carbon or the like decomposed from the machining liquid by thermal energy during the electric discharge machining on the surface of the insulating material 1. The surface of the insulating material 1 may be partially impregnated with the depositions to form said electroconductive layer. The formation of the electroconductive layer 4 may be caused by the conversion of tile surface of the insulating material 1 to an electroconductive state by thermal energy during the electric discharge machining.
The electroconductive layer 4 is expanded during the advancement of the electric discharge machining. That is, the fresh electroconductive layer 4 is formed at the part facing to the work electrode 3 where the former electroconductive layer 2 is removed, so that the fresh electroconductive layer 4 is electrically connected continuously to the remaining electroconductive layer 2, as shown in FIG. 1(D).
Consequently, even at the part facing to the work electrode 3 where the original electroconductive layer 2 has been removed, the electric discharge is continued between the work electrode 3 and the electroconductive layer 4 which is repeatedly formed, so that the insulating material is machined to a predetermined shape, as shown in FIG. 1(E).
The method shown in FIG. 1 requires special equipment and steps, resulting in the rising of machining cost, in order to preform the electroconductive layer 2 on the surface of the insulating material 1 to be machined. The electroconductive layer 4 is formed to sufficient thickness in the case of relatively shallower machining, However, when the machining depth reaches a few mm, the electroconductive layer 4 is not formed to sufficient thickness enough to continue the electric discharge machining. Hereby, there is a restriction on the depth to be formed by said method.
In order to avoid the restriction on the machining depth, we have proposed a new method as disclosed in Japanese Patent Application 5-286769. According to the proposed method, insulating and electroconductive materials are used as coupled workpieces, and the materials are electric discharge machined while relatively shifting a work electrode along a direction parallel to the matching face between the materials.
In this case, electric discharge is generated between the electroconductive material and the work electrode at first. The insulating material is machined by thermal effect at the part closest to the electroconductive material, and a component of the work electrode is transferred to said part. Other electric discharge is generated between the transferred part and the work electrode, so that the insulating material is machined by the impact of the electric discharge and the thermal effect. Due to the repetition of said generation of electric discharge and the transfer of the electrode component, the insulating material can be machined to a predetermined shape without any restriction on machining depth.
According to the former proposed method, the electroconductive layer necessary for the continuation of electric discharge machining is formed on the surface of the insulating material, even when the insulating material is machined with large depth. However, the formation of the electroconductive layer is not so rapid, since the reactions to transfer the electroconductive material and to deposit the electroconductive material from the machining liquid are relatively slow. Herein, it is necessary to machine the insulating material until the formation of the electroconductive layer having sufficient thickness, so that the insulating material is inevitably machined at a low speed. If the machining speed is elevated, there appears the tendency to reduce machining accuracy. In addition, the machining conditions are complicated and difficult to control, since the work electrode is necessarily carried along the direction parallel to the matching face of the materials.